Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”).
After the latent image is formed, toner particles are given a charge substantially opposite to the charge of the latent image, and brought into the vicinity of the photoreceptor so as to be attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles (e.g. clear toner).
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (“fuse”) the print image to the receiver. Plural print images, e.g. of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image on the receiver.
Electrophotographic (EP) printers typically transport the receiver past the photoreceptor to form the print image. The direction of travel of the receiver is referred to as the slow-scan or process direction. This is typically the vertical (Y) direction of a portrait-oriented receiver. The direction perpendicular to the slow-scan direction is referred to as the fast-scan or cross-process direction, and is typically the horizontal (X) direction of a portrait-oriented receiver. “Scan” does not imply that any components are moving or scanning across the receiver; the terminology is conventional in the art.
To accommodate hardware limitations and reduce noise, EP printers typically use screened patterns (e.g. halftones) rather than continuous tones. Marks on the receiver are placed according to a variety of geometrical patterns so that a group of marks, when seen by the eye, gives a rendition of a desired intermediate color tone between the color of the background (e.g. paper stock) and the color of the mark. U.S. Pat. No. 5,485,289 to Curry, and commonly assigned EP 0 892 549 B1 to Tai et al., describe various methods for halftoning and designing screening patterns.
Tai et al. also recognize that EP printheads have non-uniformities. For example, non-uniform exposure of an area intended to be constant density on the receiver can result in a streak, an area unintentionally exposed differently than its surround, extending in the slow-scan direction. Some streaks are consistently lighter or darker than their surrounds, corresponding e.g. to consistent over- or under-exposure on the photoreceptor. However, some streaks are lighter than their surround in some areas and darker than their surround in other areas. There is a need to compensate for both types of streaks.
U.S. Patent Application Publication No. 2005/0036705 to Viassolo et al., and U.S. Pat. No. 7,038,816 to Klassen et al., describe systems to reduce streaking by adjusting tone reproduction curve (TRC) values. However, adjusting TRC values confounds streaking-reduction with the intended purpose of TRC values, which is compensating for device non-linearities. This can increase memory requirements of a printer and restrict the available compensation to the range of adjustment provided by the TRC.
U.S. Patent Application Publication No. 2005/0134624 to Mizes describes various test patterns that can be printed on a receiver and scanned to determine streaking-compensation values. “Scanner-based technique to adjust LED printbar uniformity” by Mizes et al. (IS&T NIP19 pp. 532-536, ISBN 0-89208-247-X, dated Sep. 28, 2003) also describes test patterns and schemes for compensation. “Automatic density control for increased print uniformity and printer reliability with inline linear array sensing” by Mizes et al. (IS&T NIP24 pp. 206-210, ISBN 978-0-89208-279-7, dated Sep. 6, 2008) describes capturing an image of a test pattern strip to perform compensation. However, these schemes use TRCs for compensation, so suffer from the same limitations as Viassolo et al. and Klassen et al. Additionally, Mizes et al. [NIP 19] disclose that some observers can perceive density variations with a peak-to-peak amplitude of only 0.25ΔL*. However, Mizes et al. require extensive and time-consuming measurements to reach high precision.
There is a continuing need, therefore, for an improved way of compensating for streaks and other non-uniformities in a hardcopy reproduction apparatus which uses screening.